In the case of fracture or other injury to bone, proper bone healing and subsequent favorable bone remodeling is highly dependent on maintaining stability between bone fragments and, in the case of decalcified bone, on maintaining physiologic strain levels. External structural support can be gained using external braces, casts and the like. Internal structural support commonly is supplied by internal fixation devices such as bone plates, screws, intramedullar rods, etc., some of which may need to be surgically removed at a later time and all of which may prove to be burdensome and traumatic to a patient.
There is thus a need for a product that is a bone substitute product that is a bone graft material and that also provides structural support. This is especially so in the replacement or repair of long bones of the lower extremities and for use in spinal fusion techniques. Trauma, osteoporosis, severe osteo arthritis or rheumatoid arthritis, joint replacement, and bone cancers may call for treatment involving the use of structural bone substitute materials.
A successful bone graft requires an osteoconductive matrix providing a scaffold for bone ingrowth, osteoinductive factors providing chemical agents that induce bone regeneration and repair, osteogenic cells providing the basic building blocks for bone regeneration by their ability to differentiate into osteoblasts and osteoclasts, and structural integrity provided to the graft site suitable for the loads to be carried by the graft.
Current bone graft materials include autografts (the use of bone from the patient), allografts (the use of cadaver bone), and a variety of artificial or synthetic bone substitute materials. Autografts grafts are comprised of cancellous bone and/or cortical bone. Cancellous bone grafts provide virtually no structural integrity. Bone strength increases as the graft incorporates and new bone is laid down. For cortical bone, the graft initially provides some structural strength. However, as the graft is incorporated by the host bone, nonviable bone is removed by resorption significantly reducing the strength of the graft. The use of autograft bone may result in severe patient pain at the harvest site, and there is of course a limit to the amount of such bone that can be harvested from the patient. Allografts are similar to autografts in that they are comprised of cancellous and/or cortical bone with greater quantities and sizes being available. Sterilization techniques for allografts may compromise the structural and biochemical properties of the graft. The use of autograft bone bears at least some risk of transfer of disease and the risk that the graft may not be well incorporated.
For structural bone repair materials to be conveniently used, they must be capable of being formed into complex shapes that are designed to fit the contours of the repair site. An accurately contoured graft will enhance the integration of natural bone and provide better load carrying capability. Intimate, load carrying contact often is required between the natural bone and the bone substitute material to promote bone remodeling and regeneration leading to incorporation of the graft by host bone.
A general overview of orthopedic implantable materials is given in Damien, Christopher J., and Parsons, Russell J., "Bone Graft and Bone Graft Substitutes: A Review of Current Technology and Applications," Journal of Applied Biomaterials. Vol. 2. pp. 187-208 (1991).
A variety of materials have been proposed for use as bone substitute materials, ranging from shaped porous metal objects suitable for defect filling around knee and hip joint replacements on the one hand to shaped ceramic materials on the other. Ceramic materials by and large have been formed through a sintering process in which a powder of a ceramic material such as zirconia is compressed to a desired shape in a mold and is then heated to sintering temperatures. The porosity of the resulting material is commonly quite low. Materials employing calcium phosphates (for example: fluorapatite, hydroxyapatite, and tricalcium phosphate) can also be sintered in this manner, the calcium phosphate having the capacity for acting as a substrate for bone growth (osteoconductivity).
It has been suggested to mix ceramic powders such as zirconia and hydroxyapatite, and fluorapatite and spinel, and then compress the mixture in a mold and either sinter or hot isostatically press to produce a somewhat porous ceramic of zirconia having pores at least partially filled with hydroxyapatite. Reference is made to Tamari et al., U.S. Pat. No. 4,957,509, and also Aksaci, D. et al., Porous Fluorapatite/spinel Osteoceramic for Bone Bridges, Ceramic Transactions, Vol. 48 p. 283 (1995). It has also been suggested to use ceramic articles having both high porosity and low porosity portions, and reference is made here to Hakamatsuka et al., U.S. Pat. No. 5,152,791, Johansson, U.S. Pat. No. 5,464,440 and Borom, U.S. Pat. No. 4,237,559. See also Klawitter et al. U.S. Pat. No. 4,000,525. The latter reference refers to the use of an Al.sub.2 O.sub.3 slip that is foamed into a sponge, followed by firing.
By and large, metal or ceramic materials that have been proposed for bone substitutes have been of low porosity. The art contains examples of substantially dense metals and ceramics with a semi-porous surface which is filled or coated with a calcium phosphate based material. The resulting structure has a dense metal or ceramic core and a surface which is a composite of the core material and a calcium phosphate, or a surface which is essentially a calcium phosphate. The bone substitute materials of this type commonly are heavy and dense, and often are significantly stiffer in structure than bone. Reference here is made to U.S. Pat. Nos. 5,306,673 (Hermansson et al.), 4,599,085 (Riess et al.), 4,626,392 (Kondo et al.), and 4,967,509 (Tamari et al.).